Wideband if image rejecting receiver

ABSTRACT

A system and method for effecting wideband image rejection. In a receiver implementation, the inventive method includes the steps of receiving a first signal in a first frequency band and generating in-phase and quadrature signals therefrom. The phase of the in-phase signal is shifted to provide a second signal and the phase of the quadrature signal is shifted to provide a third signal. A predetermined phase relationship is thereby effected between the second and the third signals. The second and third signals are then summed to provide an output signal which has minimal interference from a mixing signal. In an illustrative receiver application, the phase shifting is achieved via the use of all pass networks. Each of the all pass networks include a differential amplifier having first and second input terminals. The first and the second terminals are connected to a first end of first and second resistive elements, respectively. The second ends of the first and second resistive elements are connected to a common input terminal for the network. The first input terminal is a negative terminal and is connected to an output terminal of the network. The second terminal is a positive terminal and is connected to a source of ground potential via a capacitive element.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to communications systems. Morespecifically, the present invention relates to systems and techniquesfor receiving signals over a wide range while preventing interferencefrom intermediate frequency (IF) images with desired signals.

[0003] 2. Description of the Related Art

[0004] Reception of electromagnetic signals is effected by atransmission and reception of signals in a first relatively highfrequency band (such as radio frequency or ‘RF’) and subsequentdownconversion of the received signal to a lower frequency band (such asIF.) Downconversion is typically achieved by mixing the received signalwith a reference signal generated by a local oscillator (LO). Forexample, an RF signal received at a frequency of 1000 kilohertz (kHz)might be mixed down to 100 kHz by a mixer having a 900 kHz referencesignal from a local oscillator. Under these conditions, a signal at 800kHz would have the same IF (intermediate frequency) and would thereforeinterfere with the desired signal at 1000 kHz. When mixing two signals,provision must be made to prevent the mixing image from interfering withthe desired signal. At least three techniques are known in the art forminimizing the interfering effects of mixing images.

[0005] A first technique involves the use of a narrowband filter infront of the mixer. Unfortunately, this approach suffers from therequirement of a customized design for each desired frequency ofoperation.

[0006] A second approach involves the use of tuned amplifiers, insteadof filters, in front of the mixer. Unfortunately, it has been difficultto achieve consistent gain and bandwidth rejection over a wide range ina tuned amplifier. In addition, slight variations in componentcharacteristics between amplifiers disposed in separate paths in themixer impede the ability of the amplifiers to track each other withsufficient accuracy for many demanding applications.

[0007] In accordance with a third approach, image rejecting mixers usingpassive phase shifters are used in place of filters. Passive phaseshifting networks shift phase over a single frequency or a narrow bandof frequencies. Subsequently, changing the output frequency requires aredesign or retuning of the network, both of which add cost andcomplexity to the system. This is particularly problematic in afrequency-hopping environment where external tuning networks could beused but would slow the speed at which frequency hopping would bepermitted.

[0008] Hence, a need exists in the art for an improved, yet inexpensivesystem or technique for rejecting mixing images in radio frequencyreceivers over a wide frequency range.

SUMMARY OF THE INVENTION

[0009] The need in the art is addressed by the system and method of thepresent invention for effecting wideband image rejection. In a receiverimplementation, the inventive method includes the steps of receiving afirst signal in a first frequency band and generating in-phase andquadrature signals therefrom. The phase of the in-phase signal isshifted to provide a second signal and the phase of the quadraturesignal is shifted to provide a third signal. A predetermined phaserelationship is thereby effected between the second and the thirdsignals. The second and third signals are then summed to provide anoutput signal which has minimal interference from a mixing signal.

[0010] In the illustrative embodiment, the phase shifting is achievedvia the use of all pass networks. In an illustrative implementation,each of the all pass networks include a differential amplifier havingfirst and second input terminals. The first and the second terminals areconnected to a first end of first and second resistors, respectively.The second ends of the first and second resistors are connected to acommon input terminal for the network. The first input terminal is anegative terminal and is connected to an output terminal of the network.The second terminal is a positive terminal and is connected to a sourceof ground potential via a capacitive element.

[0011] Whether implemented in a receiver or as a general-purpose mixer,the inventive mixer offers a wide-band output stage and affords goodgain matching over a wide bandwidth. This allows good image rejectionover a wide bandwidth. Hence, the inventive mixer may be used inhigh-speed frequency hopping applications without substantial changes inthe design thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a block diagram of a typical conventional implementationof an image rejecting receiver.

[0013]FIG. 2 is a block diagram of an illustrative implementation of areceiver in accordance with the present teachings.

[0014]FIG. 3, is an illustrative implementation of an all pass network(APN) utilized in the present invention.

[0015]FIG. 4 is a graph of output frequency versus gain and rejectionratio for an illustrative implementation of a receiver constructed inaccordance with the teachings of the present invention.

DESCRIPTION OF THE INVENTION

[0016] Illustrative embodiments and exemplary applications will now bedescribed with reference to the accompanying drawings to disclose theadvantageous teachings of the present invention.

[0017] While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the invention is not limited thereto. Those havingordinary skill in the art and access to the teachings provided hereinwill recognize additional modifications, applications, and embodimentswithin the scope thereof and additional fields in which the presentinvention would be of significant utility.

[0018]FIG. 1 is a block diagram of a typical conventional implementationof an image rejecting receiver. The receiver 10′ includes an antenna 12′which provides a received signal to a Low Noise Amplifier (LNA) 14′. TheLNA14′ outputs an RF (radio frequency) signal to first and second mixers16′ and 17′, respectively. A local oscillator 18′ provides a referencesignal to the second mixer 17′. This signal is shifted 90° by a firstphase shifter 19′. The output of the first phase shifter 19′ provides areference signal for the first mixer 16′.

[0019] In response, the first and second mixers 16′ and 17′ outputin-phase and quadrature IF (intermediate frequency) signals,respectively. The in-phase and quadrature signals are input to first andsecond low pass filters 20′ and 22′, respectively. The output of thefirst low pass filter is input to a second phase shifter 24′. Theoutputs of the second phase shifter 24′ and the second low pass filter22′ are summed by a summer 30′ to provide an output signal for thereceiver 10′.

[0020] Due to the relative phase shifting of the two signals, anyfrequencies below the local oscillator frequency are attenuated relativeto the frequencies above the local oscillator frequency. The amount ofattenuation depends on how accurately the 90° phase shifts are and howaccurately the gain match is between the two paths.

[0021] The 90° phase shift is conventionally performed using an RC(resistive-capacitive) filter. Since the frequency of the localoscillator 18′ stays constant and the mixer gain is insensitive to smalllocal oscillator amplitude mismatches, acquisition of a 90° phase shiftis not problematic at that frequency. In the IF stage, the gain in thetwo paths must match substantially exactly to obtain good imagerejection. However, simple RC filters will only have matching gain and a90° phase shift at one frequency. Hence, good image rejection may onlybe obtained at one frequency. As mentioned above, this may beproblematic for some current, more demanding applications. Hence, a needhas existed in the art for an improved, yet inexpensive system ortechnique for rejecting mixing images in radio frequency receivers overa wide frequency range. This need is addressed by the system and methodfor effecting wideband image rejection of the present invention.

[0022]FIG. 2 is a block diagram of an illustrative implementation of areceiver in accordance with the present teachings. As per theconventional receiver 10′ of FIG. 1, the inventive receiver 10 includesan antenna 12 which provides a received signal to a LNA 14. The LNA 14outputs an RF (radio frequency) signal to first and second mixers 16 and17, respectively. A local oscillator 18 provides a reference signal tothe second mixer 17. This signal is shifted 90° by a first phase shifter19. The output of the first phase shifter 19 provides a reference signalfor the first mixer 16.

[0023] In response, the first and second mixers 16 and 17 outputin-phase and quadrature IF (intermediate frequency) signals,respectively. The first mixer actually output the Quadrature signal. Thein-phase and quadrature signals are input to first and second all passnetworks 20 and 22, respectively. The output of the first all passnetwork 20 is input to a third all pass network 24. The outputs of thesecond all pass network 22 is input to a fourth all pass network 26. Theoutputs of the third and fourth all pass networks 24 and 26 are summedby a summer 30 to provide an output signal for the inventive receiver10.

[0024]FIG. 3, is an illustrative implementation of an all pass network(APN) utilized in the present invention. As the all pass networks (APNs)of FIG. 2 are of identical design (albeit with different resistor andcapacitor value), only one all pass network (APN) is detailed in FIG. 3.The all pass network (APN) 20 includes a differential amplifier 21. Theamplifier 21 has first and second input terminals being connected to afirst end of first and second resistors 23 and 25, respectively. Asecond end of the first and second resistors being connected to a commoninput terminal for the network. The first input terminal of theamplifier 21 is a negative terminal and is connected to an outputterminal thereof via a third resistive element 27. The second inputterminal of the amplifier 21 is a positive terminal and is connected toa source of ground potential via a capacitive element 29. Resistors foreach all pass network (APN) need to be made so that their ratios remainconstant with process variations. Capacitors also have the samerequirements for each all pass network (APN). The RC product of each allpass network (APN) needs to be staggered correctly to obtain the widestbandwidth with constant 90° phase difference with minimal error from90°. The amplifiers should be designed so that all four give identicalgains and very little phase shift at the output frequencies.

[0025] Returning to FIG. 2, the frequency of the local oscillator 18 isset below the frequency of the desired signal. The equation below setsforth a mathematical basis for practicing the present invention.

IRR=−20*log(sqrt((1+G?2−2*G* cos (phi+theta))/(1+G?2+2*G* cos(phi-theta))))

[0026] where:

[0027] G=the gain mismatch between the in-phase and quadrature paths indecimal.

[0028] phi=deviation from 90° phase delay in the quadrature path.

[0029] theta=deviation from quadrature in the LO's

[0030] Hence, the use of all pass networks (APNs) affords a constantrelative phase shift with constant gain versus frequency. As illustratedabove, two all pass networks (APNs), one in each path, can be used toobtain a 90° relative phase shift over a certain bandwidth. The all passnetwork (APN) in one path provides a 45° phase shift while the all passnetwork (APN) in the second path provides a 135° phase shift for arelative phase shift of 90°. By cascading two sets of all pass networks(APNs) as illustrated in FIG. 2, a 90° relative phase shift can bemaintained over a wider bandwidth. For example, four all pass networks(APNs) can be used to obtain 90° of phase shift over a range of 10 MHzto 50 MHz. Using the present teachings, a receiver may be implemented toprovide over 20 dB of image rejection from five MHz to over 100 MHz asdepicted in FIG. 4.

[0031]FIG. 4 is a graph of output frequency versus gain and rejectionratio for an illustrative implementation of a receiver constructed inaccordance with the teachings of the present invention. The keyparameters of a receiver implemented in accordance with the presentteachings and yielding the performance depicted in FIG. 4 are:

[0032] a) Output frequency range, from Flow to Fhigh.

[0033] b) Desired output phase ripple (total peak-to-peak deviation from90° phase shift).

[0034] c) Number of pole-zero pairs needed to obtain ‘b’ over thefrequency range in ‘a’.

[0035] From these numbers the pole-zero frequencies can be calculated,and from them and considerations set forth above, the actual resistorand capacitor values can be calculated.

[0036] Thus, the present invention has been described herein withreference to a particular embodiment for a particular application. Thosehaving ordinary skill in the art and access to the present teachingswill recognize additional modifications applications and embodimentswithin the scope thereof. For example, the inventive teachings are notlimited to use in a receiver. A mixer constructed in accordance with theteachings of the present invention may be utilized in any of a number ofapplications in which image rejection over a wide range of frequenciesis desired.

[0037] It is therefore intended by the appended claims to cover any andall such applications, modifications and embodiments within the scope ofthe present invention.

[0038] Accordingly,

What is claimed is:
 1. A wideband image rejecting receiver comprising: first means for receiving a first signal in a first frequency band; second means for generating in-phase and quadrature signals from said first received signal; third means including an all pass network for shifting the phase of said in-phase signal to provide a second signal; fourth means including an all pass network for shifting the phase of said quadrature signal to provide a third signal, whereby said second signal has a predetermined phase shift relative to said third signal; and fifth means for summing said second and said third signals.
 2. The invention of claim 1 wherein said third means includes means for shifting the phase of said in-phase signal by 45 degrees.
 3. The invention of claim 2 wherein said fourth means includes means for shifting the phase of said quadrature signal by 135 degrees.
 4. The invention of claim 1 wherein said third means includes first and second all pass networks.
 5. The invention of claim 1 wherein said fourth means includes first and second all pass networks.
 6. The invention of claim 1 wherein each of said all pass networks include a differential amplifier having first and second input terminals, said first and said second terminals being connected to a first end of first and second resistive elements, respectively, a second end of said first and second resistive elements being connected to a common input terminal for said network, said first input terminal being a negative terminal and being connected to an output terminal of said network via a third resistive element and said second terminal being a positive terminal and being connected to a source of ground potential via a capacitive element.
 7. A wideband image rejecting receiver comprising: first means for receiving a first signal in a first frequency band; second means for generating in-phase and quadrature signals from said first received signal; third means including an all pass network for shifting the phase of said in-phase signal to provide a second signal; fourth means including an all pass network for shifting the phase of said quadrature signal to provide a third signal, whereby said second signal has a 90 degree phase shift relative to said third signal; and fifth means for summing said second and said third signals; each of said all pass networks including a differential amplifier having first and second input terminals, said first and said second terminals being connected to a first end of first and second resistive elements, respectively, a second end of said first and second resistive elements being connected to a common input terminal for said network, said first input terminal being a negative terminal and being connected to an output terminal of said network and said second terminal being a positive terminal and being connected to a source of ground potential via a capacitive element.
 8. The invention of claim 7 wherein said third means includes means for shifting the phase of said in-phase signal by 45 degrees.
 9. The invention of claim 8 wherein said fourth means includes means for shifting the phase of said quadrature signal by 135 degrees.
 10. The invention of claim 7 wherein said third means includes first and second all pass networks.
 11. The invention of claim 7 wherein said fourth means includes first and second all pass networks.
 12. A wideband image rejecting mixer comprising: first means for providing a first signal in a first frequency band; second means for generating in-phase and quadrature signals from said first signal; third means including an all pass network for shifting the phase of said in-phase signal to provide a second signal; fourth means including an all pass network for shifting the phase of said quadrature signal to provide a third signal, whereby said second signal has a predetermined phase shift relative to said third signal; and fifth means for summing said second and said third signals.
 13. A wideband image rejecting mixer comprising: first means for providing a first signal in a first frequency band; second means for generating in-phase and quadrature signals from said first signal; a first all pass network for shifting the phase of said in-phase signal to provide a second signal; a second all pass network for shifting the phase of said quadrature signal to provide a third signal, whereby said second signal has a 90 degree phase shift relative to said third signal, each of said all pass networks including a differential amplifier having first and second input terminals, said first and said second terminals being connected to a first end of first and second resistive elements, respectively, a second end of said first and second resistive elements being connected to a common input terminal for said network, said first input terminal being a negative terminal and being connected to an output terminal of said network and said second terminal being a positive terminal and being connected to a source of ground potential via a capacitive element; and a summer for adding the outputs of said first and second all pass networks.
 14. A method for effecting wideband image rejection including the steps of: receiving a first signal in a first frequency band; generating in-phase and quadrature signals from said first received signal; shifting the phase of said in-phase signal to provide a second signal using an all pass network (APN); shifting the phase of said quadrature signal to provide a third signal using an all pass network (APN), whereby said second signal has a predetermined phase shift relative to said third signal; and summing said second and said third signals. 